The assembly (10, 10') is a real time imaging device for detecting radiation from an object field (12) which is periodic in time. A video camera (14) detects emitted and reflected radiation from the object field (12) and produces a video signal of the image and a timing signal. A processor (16) synchronously averages successive video signals and stores same in an image buffer (26) to obtain an image from which has been eliminated unsynchronous noise. Alternatively, the elimination of unsynchronous or ambient noise may be performed partially or wholly within the camera (14) prior to processing.